Materials such as GaN, AlN, and InN, which are nitride semiconductors, mixed crystals of such nitride semiconductors, or the like have a wide band gap and are thus used in high-output electron devices, short-wavelength light emitting devices, or the like. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV which is large compared to a band gap of 1.1 eV for Si and a band gap of 1.4 eV for GaAs.
The high-output electron devices include FETs (Field Effect Transistors), and particularly HEMTs (High Electron Mobility Transistors), as proposed in Japanese Laid-Open Patent Publication No. 2002-359256, for example. The HEMT using the nitride semiconductor is used for a high-output high-efficiency amplifier, a high-power switching device, or the like. More particularly, in the HEMT that uses AlGaN for an electron supply layer and GaN for a channel layer, piezoelectric polarization or the like is generated due to a distortion caused by a difference between lattice constants of AlGaN and GaN, and 2DEG (2-Dimensional Electron gas) having a high concentration is generated. For this reason, this HEMT can operate at a high voltage, and may be used for a high-efficiency switching element, a high withstand voltage power device of an electric vehicle, or the like.
Related art are also proposed in Japanese Laid-Open Patent Publications No. 2009-38392, No. 2010-109086, and No. 2011-249500, for example.
However, with respect to the HEMT using the nitride semiconductor, there are demands to enable operation of the HEMT in a high-frequency region, and studies are being made on gate electrodes having a short gate length using a short-gate technique. In general, in order to operate the semiconductor device in the high-frequency region, ON-resistance of the semiconductor device is preferably low.